Insulated gate bipolar transistors (IGBT) are semiconductor devices composited by metal-oxide-semiconductor field-effect transistor (MOSFET) and bipolar junction transistor (BJT), and thus possess advantages of the two kinds of devices, viz. low driving power and fast switching speed of MOSFET, reduction of saturation pressure and large capacity of BJT as well. Thus, IGBTs are recently widely applied to fields requiring power conversion, such as AC machines, transducers, switching power supplies, lighting circuits, traction drives etc.
FIG. 1 shows an example of an existing IGBT. As shown in FIG. 1, the IGBT 10 is shown to have a trench gate field stop structure, which comprises a p-type collector region 11, an n-type field stop region 12, an n−-type drift region 13, a p-type base region 14 and an n+-type source region 15 laminated successively, and a gate 16 and a gate oxide layer 17 formed in the n−-type drift region 13, the p-type base region 14 and the n+-type source region 15.
Furthermore, in the IGBT 10 shown in FIG. 1, the gate 16 includes an upper gate 161 having a uniform section width and a lower gate 162 having a section width larger than that of the upper gate 161. Such a structure is called as partially narrow mesa (PNM) structure. An IGBT having a similar structure is disclosed in the essay Low Loss IGBT with Partially Narrow Mesa Structure (PNM-IGBT) published by Masakiyo Sumitomo et al. at the 24th International Symposium on Power Semiconductor Devices and IC (ISPSD) in 2012 and in the U.S. Pat. No. 7,800,187B2. Mesa width (width of a base region between two adjacent trench gates) can be reduced without reducing metal-semiconductor contact area by forming a partially narrow mesa structure (narrowing the base region between two adjacent trench gates) as shown by the dotted box in FIG. 1, so that the saturation voltage of the IGBT 10 is significantly reduced, and a good balance between on-state voltage and turn-off loss is obtained.
However, in case of an extremely small mesa structure on the front side of an insulated gate bipolar transistor, there is a high concentration of free charge carriers in the on-state of the device close to the front side of the transistor to obtain low saturation voltage values VCEsat. Therefore, it is necessary to reduce the emitter efficiency of the backside emitter of the insulated gate bipolar transistor to reduce power losses occurring during turn-off of the insulated gate bipolar transistor. On the other hand, a soft turn-off behavior of the insulated gate bipolar transistor is required to avoid high voltage peaks during turn-off, in particular for the case of high stray inductances. Furthermore, for a good short-circuit robustness, the emitter efficiency should not be too low at room temperature or at even lower operation temperatures to avoid or at least reduce a detrimental “Umklapp”-effect of the vertical distribution of the electrical field (also known as Kirk-effect).